In about 1957 it was found that a ceramic material suitably loaded with conductive particles exhibited a sharp rise in electrical resistance at its Curie temperature and this phenomenon was named 37 The Positive Temperature Coefficient Phenomeon". Since 1957 extensive work has been done in Positive Temperature Coefficient (PTC) materials, particularly in the area of semicrystalline polymers loaded with finely divided conductive materials, particularly carbon black. This extensive work has been directed to improving the PTC phenomenon, especially for the purpose of providing a material having a built-in temperature control such that when the temperature of the material reaches a predetermined upper limit, the material becomes so resistive that it is essentially no longer conductive. This PTC phenomenon has been employed most efficiently in the electric blanket industry to provide a grid of body heat responsive PTC material surrounding a pair of conductive wires within a suitable blanket fabric material. The PTC materials have been developed with sufficient self regulating precision to provide electrode (conductor) surrounding material having the capacity to sense and deliver heat to all parts of the body in proportion to the heat requirements at any given time or location on the blanket without the necessity of internal blanket thermostats.
In spite of the extensive work that has been done in the area of new PTC materials, as evidenced by the scores of patents and articles directed to new compositions and new theories, the PTC phenomenon is one which is to date very poorly understood. A number of theories have been proposed in an attempt to explain the conductivity phenomenon for PTC materials. One theory is that the sharp positive temperature coefficient of resistance at a predetermined temperature results from thermal expansion of the polymer/finely divided conductor matrix. This theory is based on the proposal that the conductive filler is initially spread through the polymer in a network of conductive chains and as the material is heated, the conductive filler is spread out by thermal expansion until non-conductive behavior is experienced at the crystalline melting point. Others have theorized that the PTC phenomenon is due to a loss of conduction due to the more difficult electron tunneling through large intergrain gaps between carbon filler particles upon temperature rise. This theory is based upon the premise that the PTC phenomenon is due to a critical separation distance between carbon particles in the polymer matrix at the higher temperature. Still others have theorized that the PTC phenomenon is directly related to the polymer crystallinity for a given polymer so that increased crystallinity in a particular polymer causes increased PTC anomaly. For this last theory, however, there is no correlation between degrees of crystallization and the amount of PTC phenomenon that might be experienced in different polymers.
Much of the work directed to new PTC composite materials has been directed to particular conductive materials loaded into the polymer carrier and, in particular, to carbon blacks having particular reticulate structures, resistivities and/or particle sizes--see for example the Kelly U.S. Pat. Nos. 4,277,673; 4,327,480 and 4,367,168 and the Van Konynenburg et al Pat. No. 4,237,441. The patents and literature distributed by carbon black suppliers teach that the electrical conductivity of carbon blacks depends to a great extent upon the structure of the carbon blacks and the amount of surface treatment (oxidation). It is well known that higher reticulate structure grades impart higher conductivity than low reticulate structure grades and that surface treatment (volatile content) decreases conductivity. The reticulate structure of a carbon black is generally measured by its oil (dibutyl phthalate) absorption. Higher structure grades, which have a relatively large void area, absorb more oil than lower structure grades.
Carbon blacks consist of spherical particles of elemental carbon permanently fused together during the manufacturing process to form aggregates. These aggregates are defined by particle size and surface area; aggregate size or structure (reticulate structure); and surface chemistry. The particle size of carbon blacks is the size of the individual particles which are fused together during manufacture to make the aggregate and varies inversely with the total surface area of the aggregates. The surface area of carbon black aggregates is most commonly expressed in terms of nitrogen adsorption in m.sup.2 /gram using the B.E.T. (Brunauer, Emmet, Teller) procedure well known in the art. Carbon blacks having a relatively small particle size, and therefore a relatively high aggregate surface area, exhibit better conductivity or lower volume resistivity.
The size and complexity of the carbon black aggregates is referred to as "structure" or "reticulate structure". Low structure carbon blacks consist of a relatively small number of spherical carbon particles fused together compactly during manufacture to provide a relatively small amount of void space within the aggregate. High structure carbon blacks consist of more highly branched carbon a particle chains which, when fused together during manufacture, provide a large amount of avoid space within the aggregate. The structure level of carbon blacks is measured by its oil (dibutyl phthalate) absorption. Higher structure grades of carbon blacks absorb more oil than lower structure grades because of the larger avoid volume within the aggregates.
During the manufacture of carbon blacks, some oxidation naturally occurs on the surface of the aggregates resulting in the presence of chemisorbed oxygen complexes such as carboxylic, quinonic, lactonic and phenolic groups on the aggregate surfaces. Some carbon blacks are further surface treated to provide more chemisorbed oxygen on the aggregate surfaces. These surface treated carbon blacks can be identified by their low pH, less than 4.0 and generally in the range of about 2.0 to 3.0, and/or by measuring the weight loss of dry carbon black when heated to 950.degree. C. This weight loss is referred to as "volatile content" and for surface treated carbon blacks, generally is at least 3.0 weight percent and generally in the range of about 5.0 to 10.0 weight percent. The degree to which carbon blacks impart some electrical conductivity (or lessen volume resistivity) to normally non-conductive plastics depends upon four basic properties of the carbon black: surface area, structure, porosity and surface treatment. Higher structure carbon blacks impart higher conductivity (lower volume resistivity) than lower structure grades because the long, irregularly-shaped aggregates provide a better electron path through the compound. Surface treatment, on the other hand, always causes the volume resistivity to be high (low conductivity) because the surface oxygen electrically insulates the aggregates.
One of the knowns about PTC polymeric composite materials is that the polymer must, in its final state, be partly crystalline in order to exhibit PTC behavior. Experimentation with amorphous polymers filled with conductive particles, such as carbon black, do not show any increase in resistance on heating. Polymeric matrix material having a sharp increase in resistance at a predetermined temperature (PTC material) to date have not been electrically conductive without an annealing period ranging from minutes to days. U.S. Pat. No. 3,861,029 points out that polymeric materials loaded with a sufficiently high percentage of carbon black to produce a conductive material when first prepared exhibit inferior flexibility, elongation, crack resistance and undesirably low resistivity when brought to peak temperatures. Accordingly, it has been necessary to limit the carbon black content of the polymeric matrix and to anneal (heat treat at or above the crystalline melting point) for a period of time to slowly develop crystallinity until the material reaches a constant room temperature resistance. In order to provide an adequate degree of crystallinity in the polymeric matrix materials, after melting and extrusion, it has been necessary to anneal the material for a sufficient time in order to allow the required translational and conformational reorganizations necessary to fit the molecules into the properly ordered crystalline lattice structure of the polymeric material.
It is also known that the use of highly conductive carbon blacks results in a material requiring rigorous annealing to achieve a constant resistance, or results in compositions having resistances too high to be of practical use. The prior art compositions, however, have required the highly conductive carbon blacks to achieve a composition having sufficient electrical conductivity and exhibiting PTC behavior. As disclosed in the Kelly U.S. Pat. Nos. 4,277,673 and 4,327,4890 and 4,367,168, the use of highly resistive (essentially non-conductive) carbon blacks such as the surface treated Mogul L and Raven 1255, when used in the range of 5 to 15%, substantially reduces the necessary annealing time down to a period of about two to three hours in some cases.
It has been found that the selection of polymers having a suitable number of relatively low molecular weight molecules, together with a carbon black having essentially no chemical surface treatment (oxidation), as indicated by a pH of at least 4.0, and generally in the range of 5.0 to 8.5, and having a relatively low reticulate structure, as defined by the relation between nitrogen surface area and DBP absorption according to the following equation: A.gtoreq.1.75x+e.sup.x/37 where A=nitrogen surface area in m.sup.2 /gram and x=DBP absorption in cc/100 grams, substantially eliminates the annealing time necessary for the PTC material to achieve a substantially constant room temperature electrical resistance.